1. Field of the Invention
The present invention relates to methods of manufacturing electronic devices, and more particularly, to a method of manufacturing an electronic device in which a communication element, such as a surface acoustic wave filter, a piezoelectric filter using piezoelectric thin-film resonators, an optical signal modulator, or a high-frequency element, is flip-chip mounted on a bond substrate and is then hermetically sealed.
2. Description of the Related Art
In recent years, in accordance with the trend to make electronic apparatuses compact and light weight, multifunctionality has been required for electronic devices. According to the trend described above, piezoelectric devices used for communication apparatuses including mobile phones, such as a surface acoustic wave filter (hereinafter referred to as a “SAW filter”) and a piezoelectric filter (hereinafter referred to as a Bulk Acoustic Wave Filter or “BAW filter”) including piezoelectric resonators, have also been required to be compact and light-weight.
The BAW filter includes piezoelectric resonators arranged in a ladder pattern or in a lattice pattern. As the piezoelectric resonator, for example, a piezoelectric resonator including a vibration portion, which includes a thin-film composed of at least one piezoelectric thin film (formed of, for example, ZnO or AlN), and at least one pair of a top electrode and a bottom electrode, which face each other with the thin-film provided therebetween, is provided over an opening or a concave portion provided in an Si substrate, or a piezoelectric resonator in which a space is provided between a bottom electrode and an Si substrate which is not provided with an opening or a concave portion. In the piezoelectric resonators described above, since vertical vibration generated in the vibration portion is used, a vibration space must be provided, and in addition, the vibration portion must be protected from moisture, dust, and other contaminants.
In addition, a SAW filter includes a piezoelectric substrate composed, for example, of quartz, LiTaO3, or LiNbO3, and a pair of comb-shaped electrode portions (interdigital transducer, hereinafter referred to as an “IDT”) provided thereon, which are made of a metal such as Al. In the surface acoustic wave filter described above, a vibration space for propagation of a surface acoustic wave must be provided at the comb-shaped electrode portions and the piezoelectric substrate, and in addition, the comb-shaped electrode portions must be protected from moisture, dust, and other contaminants.
The BAW filter and the SAW filter described above are each typically formed by the steps of applying a die-bonding agent on the bottom surface of a ceramic package made of alumina or other suitable material, mounting an element for the BAW filter or the SAW filter on the package via die-bonding, connecting electrodes of the element to terminals provided inside the package via wire bonding, and sealing the package with a lid. In addition, to reduce the size of the BAW filter and the SAW filter, a method has also been performed including the steps of forming electrode lands on the bottom surface of a package made of alumina or other suitable material, mounting an element for the BAW filter or the SAW filter on the package by flip-chip bonding, and sealing the package with a lid.
However, according to the structure described above, although the size of the elements for the BAW filter and the SAW filter are reduced, when the size of the package is not reduced, the sizes and heights of the BAW filter and the SAW filter cannot be reduced. In addition, that cost of forming a small package is high. Furthermore, in particular for the BAW filter, since the vibration portion is provided in an opening or in a concave portion of the substrate, the vibration portion is damaged by impact generated in a step in which an element is processed by dicing, is picked up for mounting, or is die-bonded. Accordingly, recently, a chip size package (hereinafter referred to as “CSP”) BAW filter and a CSP SAW filter have been proposed.
In a conventional CSP SAW filter, an element (SAW element) for the SAW filter is mounted on a substrate. When this SAW element is mounted on the substrate and is then packaged, the following conditions must be satisfied.
(1) A hermetically sealed structure in which an IDT (primarily composed of Al) defining the SAW element is isolated from an ambient atmosphere for preventing corrosion.
(2) Electrical conduction between electrodes of the SAW element and the bond substrate is required.
A mounting structure of a surface acoustic wave device which satisfies the above-described conditions is disclosed in Japanese Unexamined Patent Application Publication Nos. 4-293310 and 2000-77970.
In Japanese Unexamined Patent Application Publication No. 4-293310, a surface acoustic wave device includes a SAW element that is flip-chip bonded to a bond substrate with bumps provided therebetween, and a sealing frame surrounding IDT electrodes and the bumps is bonded to the bond substrate so as to provide a vibration space and to achieve a hermetic seal. In the case described above, the bumps and the sealing frame are both composed of solder and are simultaneously formed by applying cream solder on the bond substrate by screen printing, followed by melting and solidifying the solder. After the SAW element is arranged on the bond substrate such that input-output electrodes of the SAW element are brought into contact with the bumps, the bumps are bonded to the input-output electrodes by applying heat and pressure, and at the same time, the solder sealing frame is bonded to a seal ring provided at the element side.
According to Japanese Unexamined Patent Application Publication No. 2000-77970, similar to the structure described above, a SAW element is flip-chip bonded to a bond substrate, and a sealing frame surrounding IDT electrodes and bumps is bonded to the bond substrate so as to provide hermetic seal. That is, input-output electrodes and a seal ring are provided on a surface of the SAW element, lead electrodes and a seal ring are provided on the bond substrate, bumps are provided on the lead electrodes, and a solder sealing frame is provided on the seal ring at the substrate side. The bumps are formed on the lead electrodes by a wire bonding method, and the solder sealing frame is formed by the steps of printing a solder paste on the seal ring at the substrate side, and reflowing the solder paste followed by washing.
According to Japanese Unexamined Patent Application Publication Nos. 4-293310 and 2000-77970, the following problems may arise since the solder sealing frame is formed at the bond substrate side, and a printing method is used therefor.
Since the solder sealing frame is formed on the bond substrate which is a portion of a mother substrate including a plurality of the bond substrates, when solder is printed on the substrate, cumulative pitch accuracy of the electrodes formed on the mother substrate and the accuracy of a metal mask for printing must be consistent with each other, and as a result, it becomes difficult to form a finer sealing frame. Hence, the size of an element is increased, and the number of the elements formed on the mother substrate is decreased.
In addition, since a printing amount of solder that is generated in printing varies due to warping of the mother substrate and variation in electrode thickness, the height of the bumps varies after reflowing, and as a result, bonding defects occur. Furthermore, in reflowing, since the solder aggregates at corner portions to form convex shapes, the height of the solder varies, and as a result, bonding defects occur.
In order to solve the problems described above, a method of forming a sealing frame may be used in which a solder paste is printed on a wafer having a number of elements formed thereon followed by reflowing. However, in the method described above, the accuracy of the solder sealing frame cannot be effectively obtained, performance is degraded due to damage to electrodes on the surface of the wafer caused by a printing mask, the wafer is broken by a squeegee printing pressure applied in printing or by heat applied for reflowing the solder, and other defects are likely to occur.
In Japanese Unexamined Patent Application Publication No. 9-162690, a method is disclosed in which a SAW element is flip-chip bonded to a bond substrate, and a sealing frame surrounding IDT electrodes and bumps is bonded to the bond substrate so as to provide a hermetic seal. That is, input-output electrodes and a seal ring are formed on the surface of the SAW element by a lift-off technique, and the bumps and the sealing frame are formed on the input-output electrodes and the seal ring, respectively, by a lift-off technique.
The bumps and the sealing frame each have a two-layered structure, for example, of nickel and gold. In addition, lead electrodes and a seal ring are provided on the bond substrate. The seal ring on the bond substrate is formed, for example, by performing seal printing of gold. Subsequently, the SAW element is preliminarily pressed onto the bond substrate and is then bonded thereto by reflowing.
In the case disclosed in Japanese Unexamined Patent Application Publication No. 9-162690, since a lift-off technique is used instead of a printing method, the problems described above do not occur. However, since gold having a melting point higher than that of solder is used for the bumps and the sealing frame, the temperature is significantly increased in reflowing, and properties of the SAW element are degraded. In addition, since a large amount of gold is used, the cost is increased substantially.
In addition, when gold having a predetermined thickness is processed by a lift-off technique, since stress generated when a film is formed using gold is large, a resist pattern used for the lift-off is considerably deformed, and as a result, deviation in the patterns of the bumps and the sealing frame often occur.
Furthermore, since a seal ring is provided on the surface of the SAW element in advance, and the sealing frame is formed thereon, extremely high alignment accuracy is required, and in addition, the width of the seal ring must be greater than that of the sealing frame. As a result, the size of the SAW element is increased.
In addition, in a CSP BAW filter, an element (BAW element) for the BAW filter is bonded to a bond substrate. When this BAW element is mounted on the bond substrate and is then packaged, the following conditions must be satisfied.
(1) A hermetically sealed structure in which a top electrode and a bottom electrode, which define the BAW element, are isolated from an ambient atmosphere for preventing corrosion.
(2) Electrical conduction between electrodes of the BAW element and the bond substrate.
A mounted structure of a BAW device, which satisfies the above-described conditions, is disclosed in Japanese Unexamined Patent Application Publication Nos. 7-283334 and 2001-68580.
In Japanese Unexamined Patent Application Publication No. 7-283334, a BAW filter includes a BAW element that is flip-chip bonded to a bond substrate via bumps provided therebetween, and a sealing member (corresponding to the sealing frame) surrounding a vibration portion and the bumps is bonded to the bond substrate so as to provide a vibration space and a hermetic seal. In the case described above, the sealing member (corresponding to the sealing frame) is formed of a low melting point glass, such as lead glass, or cream solder.
According to Japanese Unexamined Patent Application Publication No. 7-283334, when a low melting point glass is used for the sealing member (corresponding to the sealing frame), the following problems arise. First, since a low melting point glass such as lead glass contain lead which causes environmental problems, the use thereof is not preferable. However, when glass which does not contain lead is used, the melting point is increased to approximately 400° C. or more, and as a result, the top electrode, the bottom electrode, and piezoelectric thin films of the BAW element may be damaged in a manufacturing process. Furthermore, since a low melting point glass does not have sufficient strength, when the coefficient of thermal expansion of a BAW element substrate is different from that of the bond substrate, the strain generated cannot be absorbed. On the other hand, when cream solder is used for the sealing member (corresponding to the sealing frame), problems similar to those described in Japanese Unexamined Patent Application Publication Nos. 4-293310 and 2000-77970 occur during printing.
In Japanese Unexamined Patent Application Publication No. 2001-68580, a BAW filter is disclosed in which a microcap (corresponding to the bond substrate), which is provided with a gasket (corresponding to the sealing frame) and which is used to provide a vibration portion, is bonded to a BAW element by cold pressure bonding so as to provide a vibration space and a hermetic seal. In the case described above, the gasket (corresponding to the sealing frame) is formed of gold.
As described above, according to Japanese Unexamined Patent Application Publication No. 2001-68580, the microcap (corresponding to the bond substrate) provided with the gasket (corresponding to the sealing frame) is bonded to the BAW element by cold pressure bonding. In cold pressure bonding, since considerable pressure is applied to the BAW element, problems may arise in that the vibration portion of the BAW element is destroyed or a BAW element substrate is damaged by the pressure. In addition, when the degree of flatness of the BAW element substrate is not high, it is difficult to achieve sealing by cold pressure bonding, and the yield is thus decreased. Furthermore, when a very small amount of dust is present on the BAW element substrate, it becomes difficult to uniformly apply pressure on the periphery of the substrate, and in the worst case, the wafer may be broken.